Embodiments described herein generally relate to volume rendering of medical images.
In the medical field, three-dimensional (3D) image data sets, i.e. volume data sets, are collected by a variety of techniques—referred to as modalities in the field—including computer assisted tomography (CT), magnetic resonance (MR), single photon emission computed tomography (SPECT), ultrasound and positron emission tomography (PET).
When displaying an image, such as in medical imaging applications, particular signal values are associated with particular opacities and also, in the case of non-grayscale images, colors to assist visualization. This association or mapping is performed when using data from a 3D data set (voxel data set) to compute a 2D data set (pixel data set) which represents a 2D projection of the 3D data set for display on a computer screen or other conventional 2D display apparatus. This process is known as volume rendering or more generally rendering.
There are various clinical situations in which tissue thickness is an important diagnostic metric. For example, in cardiac imaging the myocardial wall thickness is used as an indicator of hypertrophy.
FIG. 1 shows a schematic section of a healthy heart on the left and a heart with right ventricular hypertrophy on the right. As can be seen the ventrical wall V is thicker in the diseased heart.
In a volume rendering application for visualizing a 3D patient image data set, the wall thickening can be seen by taking a section through the heart as in the schematic illustration, but creating this view can be quite time consuming. However, without taking a section, it is not straightforward to visualize thickness metrics or to fuse them with the structural data.
One known method is to project the thickness measurement onto the heart wall as a texture—this is a feature of Toshiba Medical Systems CT Console.
FIG. 2 is a flow diagram explaining this method of visualizing the thickness of the myocardium.
In Step S1, segmentation of a CT volume data set of the heart is carried out to identify the compartments and other features.
In Step S2, a thickness map of the myocardium is computed.
In Step S3, the thickness is mapped to colors.
In Step S4, the modified volume data set is volume rendered using the color mapping of the thickness as a texture on the external surface of the myocardium.
In Step S5, the 2D image from the volume rendering is output for display.
This method works rather well but, since the texture is artificial, the volume rendered image appears artificial to the user.
A somewhat related method is disclosed in JP 09-237352 A for visualizing the rough internal surface of a lumen (or organ), such as the trachea, esophagus, intestines or stomach. It is proposed to modify a conventional external view of the lumen by shading the outside surface of the lumen with the properties of the inside surface of that lumen. The inside surface topography is determined by providing a virtual light source inside the feature. A line light source following the centroid of each slice of a data volume is proposed as the virtual light source, i.e. a point light source in the center of each slice. This allows the topography, i.e. roughness, of the internal surface of the stomach, trachea etc. to be measured and then, by a suitable transform, visualized on the external surface of the stomach, trachea etc. We note the authors understanding of JP 09-237352 A by Hitachi Medical Corp. is solely based on a machine translation into English from the Japanese, so may not be correct.
A common, more conventional method is not to use a rendering application to assess the myocardial viability, but instead to use a so-called polar map or bull's eye plot as proposed by E. Garcia, K. V. Train, J. Maddahi, F. Prigent, J. Friedman, J. Areeda, A. Waxman, and D. Berman, “Quantification of rotational thallium-201 myocardial tomography,” Journal of Nuclear Medicine, vol. 26, no. 1, pp. 17-26, 1985. A polar map represents the 3D volume of the left or right ventricle as a 2D circular plate divided into four concentric rings to create 17 regions in total.
FIG. 3 is a polar map of the left ventricle following the American Heart Association (AHA) recommendation. Each region of the display, corresponding to a specific region of the myocardium (see FIG. 3 for the names of the regions), receives a color according to normalized count values. The polar map has been widely used in clinical practice in the last two decades. The AHA standards allow a comparison to be made between different modalities, including SPECT. The bull's eye plot lacks the advantage of a single fused view that in principle should be achievable by volume rendering, but provides an easy way to make a diagnosis.